A common problem in the processing of mineral ores, particularly in the processing of bauxite by the Bayer process, for the production of alumina is the generation and accumulation of oxalate ions commonly referred to as oxalates in the Bayer processing streams and waste streams. Environmental concerns may prevent the dumping of oxalates at disposal sites. In view of the appreciable quantities of oxalate solids which have to be removed from the Bayer process, which may range from one to ten tons daily in a typical medium-sized Bayer processing plant, the use of large scale disposal systems are required. For example, the solid oxalates with or without liming may be buried in red mud from the Bayer processing system. The oxalates could also be burned in a kiln, such as a lime kiln; however, these procedures are by and large prohibitively expensive.
As is generally understood, bauxites by and large contain low percentages, usually less than 0.5% of organic matter mostly in the form of humates. In the Bayer processing system, the humates, which dissolve in the process liquors, are ultimately degraded to acetates, formates, carbonates and oxalates. These compounds tend to concentrate as their sodium salts in the processing liquors. Most of these sodium salts have relatively high solubilities in the processing liquors and as such do not interfere with the process. However, sodium oxalates have a relatively low solubility and readily precipitate out of the processing liquors particularly in portions of the processing system which operate at cooler temperatures and higher caustic soda concentrations. As the oxalates precipitate out of the processing solution, restrictions in the processing system result which can considerably reduce the effectiveness of the overall processing operation. Furthermore, the build-up of oxalates in the processing liquors tends to cause a reduction in alumina trihydrate yield from the process and ultimately a co-precipitation of the sodium oxalate with the alumina trihydrate. When the alumina trihydrate is contaminated with oxalates, it has been found that calcination of the precipitate results in a very weak and fine alumina which leads to excessive dust losses in the subsequent calcination step and production of aluminum by electrolysis. Hence it is very important to keep the concentration of oxalate levels in the Bayer processing system below some critical value to avoid these problems. Although it is possible and is routinely done in many areas to remove oxalates from the processing system by drawing off a portion of the liquors, or by precipitating out and removing solid sodium oxalate, the above noted alternatives in the ultimate disposal of the oxalates are becoming commercially prohibitive.
Australian patent application 39465/89 (common assignee) discloses a process for the microbial degradation of oxalate ions in a stream discharge from the Bayer processing system. The microbial degradation provides an effective economically attractive approach to the removal of oxalates from a waste stream of Bayer processes. The process can be operated on either a continuous or batch basis. It was thought necessary however to adjust the pH of the oxalate containing waste stream and to reduce sodium ion concentration in the waste stream to levels which would be compatible with the microorganisms of the bioreactor. The preferred microorganisms are of the Pseudomonas genus.
As described in international publication W092/00370 (common assignee), a particularly preferred oxalate degrading bacterial species is variant B-1 belonging to the genus Pseudomonas and having the characteristics of deposit ATCC 53883. The microorganism, in addition to its deposited information can also be characterized as being isolated from rhizosphere soil surrounding a Rhubarb plant or a Dieffenbachia plant and having the following taxonomic data:
______________________________________ i) Gram negative + Aerobic metabolism + Polar flagella + Motility + Oxidase weak and slow Catalase + ii) resembles Xylophilis ampelina iii) differs from Xylophilis ampelina in the following distinct characteristics: X. ampelina B-1 Utilization oxalate as - + sole carbon source Acid from O-F arabinose + - Urease + - Growth Factor Requirements + - ______________________________________
It was preferred that the biodegradation process using this microorganism be carried out at pH close to neutral and that the sodium ion concentration in the incoming stream did not exceed 8,000 mg/L. Furthermore, it was preferred that expensive nutrients be constantly added to the media of the bioreactor. Such nutrients included magnesium sulfate, iron sulfate and yeast extract. Such preferred processing techniques with the microorganism of deposit ATCC 53883 substantially added to the operating costs for degrading oxalates, although such steps had always been thought necessary to ensure long term continuous viability of the microorganisms in the bioreactor.
Other attempts have been made in the microbial biodegradation of oxalates in waste streams from Bayer processing systems. International publication W091/12207 discloses the microbial degradation of oxalates in the waste stream by use of alkalophilic oxalate-degrading aerobic microorganisms. Such microorganisms are of the Bacillus species and which were derived from the alkaline waters of Bayer plant cooling ponds or mud disposal ponds. Although the process of this published application is effective in biodegradation of oxalates at a pH in the range of 8 to 12, there are however significant drawbacks to the process in that acetates and benzoates and other organic materials in the oxalate rich waste stream do not appear to be degraded and furthermore, the residence time for treatment of the oxalate rich streams is in the range of 50 hours or more. This lengthy residence is necessary in order to achieve 100% degradation of the oxalates in the waste stream.
According to this invention, an improvement in the biodegration of oxalates derived from a Bayer processing system is provided. The improvement relates to the degradation of the organics which include the oxalates, acetates, benzoates and formates in the incoming waste stream at high pH in the range of 8 to 12 and also at high levels of sodium ion concentration common to those found in the oxalate containing streams. Such degradation is carried out with the Pseudomonas species of microorganisms having characteristics of ATCC 53883 and its derivatives which are functionally, biologically equivalent to the deposited material. By use of these microorganisms at the high pH, at the high sodium ion concentration and at ambient temperature, residence times for the treated stream is considerably reduced by at least 10 fold compared to prior art processes.